The present invention relates to light-absorbing materials, and, more particularly, to materials that passively protect optically sensitive materials from overexposure to high intensity light.
Optically sensitive materials such as optical sensors and the human eye are used to detect light because they contain components which are sensitive to light energy, and produce measurable reactions when exposed to light energy. In one common example, an optoelectronic sensor such as an "electric eye" produces a voltage when light falls upon a sensing material within the sensor. However, such sensors can be "blinded" by overexposure to high intensity light, just as the human eye becomes blinded if it is exposed to overly intense light. In each case, exposure to a high intensity of light can temporarily or even permanently destroy the ability of the optically sensitive material to react to light.
Many situations of potential damage to the human eye are well known. A welder uses a dark shield when the welding arc is lit, and removes the shield to inspect the job after the arc ceases. Persons working with lasers must be very careful not to expose their eyes to the direct energy of the laser beam. Optical sensors that in some senses mimic the behavior of the human eye require similar consideration. If a sensor that is monitoring a welding operation is suddenly exposed to the direct arc light without any protection, it may be damaged. Laser monitoring devices may be damaged if suddenly exposed to the full intensity of the laser beam, without proper protection.
There are two approaches to protecting such optically sensitive materials against blinding by overexposure to high intensity light. In an "active" approach, when the electronic circuitry of the optically sensitive device detects a harmfully high intensity of light, it operates a mechanism to interpose a physical barrier between the light source and the optically sensitive material. For an optoelectronic sensor, the barrier could be a solid shutter that is placed between the light source and the sensor. For the human eye, the eyelid is closed when the light intensity becomes too high. Active protection systems work well when the onset of high intensity light is fairly slow so that the protection system has time to respond, or when the interposition of the protective barrier does not pose an inconvenience. When a welder places a dark glass before his eyes as he strikes the welding arc and removes it when he finishes the weld and needs to inspect it, he is using an active protection system.
On the other hand, in some instances the onset of the harmful high-intensity light is so fast that no active system has the time to respond, and in that case a "passive" protection approach is needed. In a passive protection system, a barrier is formed directly in response to the incidence light, without the need for the operation of any device or mechanism having a delay time for response. Passive protection systems therefore do not lose effectiveness because of disablement of a mechanism, and are more convenient to use in some situations. For example, a more convenient and safer approach for the protection of a welder's eye would be a passive system such as a goggle that turns dark when the arc is struck and lightens when the arc is turned off.
An example of an existing passive protection device is photochromic sunglasses whose light absorption increases in bright light and decreases in light of lower intensity. While this type of device works well in some circumstances, it is not satisfactory in others. Photochromic transitions tend to be rather slow, typically on the order of seconds, as they depend upon the occurrence of chemical reactions within the protective material. A photochromic protective shield would not be effective as a protective barrier against an intermittently pulsed laser or for many welding situations, because the damaging intensity of light is reached within milliseconds after the initiation of the light. Thus, persons working around lasers emitting light in the visible range would not have their eyes properly protected by photochromic sunglasses, because the laser pulses would rise to the harmful range too rapidly for the sunglasses to respond. A welder would similarly be unprotected against at least the initial portions of the arc.
One potential solution to the need for better passive protective devices is found in reverse saturable absorbing (RSA) materials. When interposed in a light beam, such materials permit a fraction of incident light to pass up to some limiting value, and then limit the amount of transmitted light above that value. The functioning of RSA materials depends upon electronic transistions rather than chemical reactions, and therefore protection can be achieved in far less than a millisecond after initiation of the harmful light.
RSA materials such as Indanthrone, Sudanschwarz B, metal dithizonates, chloroaluminum phthalocyanine, and spiropyrans have been known for some time. While operable in a laboratory environment, these materials have drawbacks that limit their utilization in many practical applications. Some of the materials have a limiting transmission that is so high that the sensor may be damaged at that level. Some materials have absorption bands that limit their usefulness over a full spectrum of incident light. Others become chemically unstable even at only slightly elevated temperatures. Others cannot be processed in any reasonable fashion because they are not soluble in common solvents. Many of the candidate RSA materials suffer from more than one of these problems. As a result of these various shortcomings, to date there have been no practical applications of reverse saturable absorber materials in situations requiring protection of the eye or of other types of optical sensors.
There is a need for an improved passive protection approach and reverse saturable absorber material that is operable over a broad spectrum of visible light, is stable, and can be processed. The present invention fulfills this need, and further provides related advantages.